/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.api.java.functions;

import org.apache.flink.annotation.Internal;
import org.apache.flink.api.common.functions.RichMapPartitionFunction;
import org.apache.flink.api.java.sampling.BernoulliSampler;
import org.apache.flink.api.java.sampling.PoissonSampler;
import org.apache.flink.api.java.sampling.RandomSampler;
import org.apache.flink.util.Collector;

import java.util.Iterator;

/**
 * A map partition function wrapper for sampling algorithms with fraction, the sample algorithm
 * takes the partition iterator as input.
 *
 * @param <T>
 */
@Internal
public class SampleWithFraction<T> extends RichMapPartitionFunction<T, T> {

    private boolean withReplacement;
    private double fraction;
    private long seed;

    /**
     * Create a function instance of SampleWithFraction.
     *
     * @param withReplacement Whether element can be selected more than once.
     * @param fraction Probability that each element is selected.
     * @param seed random number generator seed.
     */
    public SampleWithFraction(boolean withReplacement, double fraction, long seed) {
        this.withReplacement = withReplacement;
        this.fraction = fraction;
        this.seed = seed;
    }

    @Override
    public void mapPartition(Iterable<T> values, Collector<T> out) throws Exception {
        RandomSampler<T> sampler;
        long seedAndIndex = seed + getRuntimeContext().getIndexOfThisSubtask();
        if (withReplacement) {
            sampler = new PoissonSampler<>(fraction, seedAndIndex);
        } else {
            sampler = new BernoulliSampler<>(fraction, seedAndIndex);
        }

        Iterator<T> sampled = sampler.sample(values.iterator());
        while (sampled.hasNext()) {
            out.collect(sampled.next());
        }
    }
}
